Aneuploidy and chromosomal instability (CIN) are remarkably common in solid tumors including breast cancers. Identifying and characterizing recurring genetic and epigenetic changes in aneuploid cancers will have tremendous impact on cancer prevention, diagnosis and treatment. TRIP13 was ranked as one of the top genes associated with CIN (#12 in the CIN70 signature), and its overexpression was also observed in breast cancers with poor prognosis in multiple microarray studies. However, the working mechanism of TRIP13 and the pathological significance of TRIP13 overexpression in breast cancer development remain unclear. Our preliminary work found that TRIP13, a member of the AAA-ATPases, is a novel kinetochore protein and directly interacts with mitotic checkpoint silencing protein p31comet. TRIP13 knockdown delayed metaphase-to- anaphase transition and significantly reduced breast cancer cell proliferation in 2D culture and soft agar plates. The objective of current proposal is to elucidate mitotic functions of TRIP13 and examine the relationship between TRIP13 overexpression and breast cancer development. The central hypothesis is that TRIP13 overexpression promotes untimely mitotic checkpoint silencing, driving CIN and cancer development in mammary epithelial cells. Three specific aims will test the central hypothesis. Aim 1 will assess biochemical functions of TRIP13 in the mitotic checkpoint. Specifically, we will test that TRIP13, through its interaction with p31comet and its ATPase activity disrupts protein complexes required for mitotic checkpoint signaling. In Aim 2, we will use inducible cell lines to modulate TRIP13 expression level and determine the impact on chromosomal stability and cell proliferation in 2D and 3D cell culture models. Aim 3 will examine the effects of TRIP13 overexpression or knockdown on breast cancer cell growth in xenografted mouse models. Together, the proposed work will test potential oncogenic functions of TRIP13 ATPase and assess the possibility to target TRIP13 as a novel means to control breast cancers. The work also addresses some critical knowledge gaps in mitotic checkpoint signaling studies. Dissecting TRIP13 functioning mechanisms represents an important step in our continuous efforts to understand and exploit aneuploidy and CIN in breast cancers.